The Ultimate Alchemist
by Spartan of Chaos
Summary: Ben Tennyson finds himself in a world based on the science of Alchemy. He crosses paths with the Elric Brothers and decides to do what he can to help them get their bodies back and stop the Homunculi. However, little do our heroes know that an even worse enemy will soon rear its ugly head.
1. Chapter 1

**_AN: After browsing through the crossover archive recently, I noticed that an actual crossover with Ben 10 and Fullmetal Alchemist hasn't been done yet considering that pretty much every other anime has been done already. So it pleases me to be the first author to do so. Omniverse never happened in this story but that doesn't mean I won't borrow any elements of that series. I decided that the only way I could make this work is by bringing in Eon and some of Ben's most dangerous foes into the mix as Ben 10 villains don't exactly get featured very much in these kinds of stories and that Ben could probably take on the Homunculi by himself. Really, that's all I have to say about this one, I'm really excited to see where it goes and hopefully, you guys can enjoy it too._**

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_**Liore**_

Dusk was settling in the town of Liore, as the riots between the loyalists and the so called heretics raged on. However, up in the belltowers of the Cathedral, something sinister was taking place. the Homunculi had been watching from one of the windows, looking down at the people below.

Humans were truly a despicable species.

A cacophony of battle cries and screams echoed throughout the town of Liore. The metallic scent of lifeblood permeated the atmosphere, mixing with smoke and ash as a result of tragic deaths. Ah, she could feel all the suffering in the air, and the anticipation of what was to come.

"Look at them all, humans are foolish beyond salvation," Lust remarked.

Generally useless too, for Cornello's incompetence and failure had forced her to take matters into her hands. Drunk with power of the Philosopher's Stone, he disobeyed her words, and turned into a flailing mess of a monkey.

Though, she was irritated by Fullmetal and his brother's interference. They were meddling little pests who kept getting in their way, and she couldn't even kill them for that as they were important human sacrifices. At least, they gave her a valid reason to get rid of Cornello once and for all.

"Foolish, foolish!" Lust patted Gluttony's head as he echoed her thoughts, watching the spectacle of the human conflict together from the church's tower. It was like a war between two factions of ants, one devoted followers of the fake god Leto and the other heathens who defied said god. She hummed in appreciation when more blood was spilled to the ground, feeling remorseless over the deaths of insects beneath them.

"Fools indeed. Sad, but true." 'Cornello' said as he walked up the stairs of the tower.

"Well, well, if it isn't the Father." Lust said.

"Father." Gluttony said.

"Sorry to have to put you to all this trouble." Lust said.

"As soon as we have everything handled over here, I have my own responsibilities I have to get back to." Envy told them.

Lust smirked and turned her head to the side. "The Fullmetal boy's interference was irritating, but at least we were able to make it work in our favor. Looks like we'll finish up ahead of schedule."

The land was overlooked with the sun starting to sink. "Manipulate a little information and I whipped up the believers, and that's all it took to get this. Humans are such simpletons."

"Bloodshed gives way to more bloodshed, hatred breeds more hatred, until all the violence soaks into the land." Lust's tattoo was shown. "Craving rivers of blood." Her face was shown as she continued. "And no matter how many times it happens, they never learn. The human race is made up of violent, miserable fools."

"Who seem more than willing to play into our hands." 'Cornello' said, causing Lust to smirk.

"Are lots of people gonna to die again?" Gluttony asked.

"Yes, they will." Lust said.

"Can I eat all of them after they die?" Gluttony asked.

Lust petted Gluttony's head. "No, you can't." 'Cornello' was shown, smirking. "By the way, Envy, how long are you planning to look like that? It's very unattractive."

The priest grinned, before red lightning bolts appeared out of his body.

"Hey, hey, I was just going with the flow," he said as his body rapidly transformed, his voice became a dissonance of a gruff and a mischievous young voices. "But I guess it's better to transform into someone young and cute, huh?"

A teenage boy was revealed, with long spikes of black hair and attire that were familiar to her. He grinned when one of Cornello's followers gasped in disbelief.

"M-m-monster!" The follower screamed. "W-what's this?! Where's the real Father Cornello?!"

"What should we say?" Lust asked.

Envy sighed, "He called me a monster. That was rude, huh?"

Gluttony appeared behind Envy, staring at the follower. "Can I eat him?"

Envy and Lust looked at him before the sounds of eating took place of the scene. While Gluttony ate him, Envy leaned against the railing of the balcony.

"Oh, hey, did you hear the news? Shou Tucker, that alchemist in East City, he's dead." Envy said.

"Tucker? Why should we care about a little nothing like him?" Lust asked.

"Because it was _him_ responsible for the murder." Envy said, "Hey Gluttony, you mind chewing with your mouth closed?" He grumbled as Gluttony looked up with a mouthful of flesh from the unfortunate priest who stumbled upon them.

"Don't bother, I doubt this pathetic creature knows anything about table manners," A voice answered from the darkness, causing Lust and Envy to spin around, ready for a fight. Instead, they were met with a pair of glowing purple eyes illuminating in the dark. "Who the hell are you?" Envy demanded, the figure then approached the light. Revealing a man with a muscular build. He wore a silver helmet with a purple visor that blocked all view of his face, leaving only two glowing purple eyes visible. He wore a black bodysuit, and had silver boots, gauntlets, and a chest plate, a purple cape was also hanging off his shoulders. The duo's eyes widened. "What?!" Envy demanded, voice seething with rage and frustration, "Where did you come from?!"

"Where, you may ask?" The armored man repeated in bemusement, "Somewhere your feeble mind couldn't hope to comprehend," Both homunculi turned to see that the surrounding area was actually aging into dust in this being's presence much to the homunculi's shock and disbelief. "I've been wanting to visit this world for quite some time," the figure spoke in a condescending tone, "Do not worry. I'm not here to hurt you...for now."

Envy growled at him and was ready to respond with a biting remark, but Lust stopped him, giving him a look of warning. "Who are you?" she demanded, "What is it that you want?"

"I suppose introductions are in order," the figure responded, "You may call me Eon, Master of Time, and me and associates would like to discuss some business with the one you call...Father or should I say Dwarf in the Flask?" The two homunculi looked at each other for a moment as if considering their options silently. Finally, they nodded and decided to listen. It was probably good for their health if they didn't decide to test this stranger's patience...

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_**AN: Hope you enjoyed the prologue as the next chapter will deal with Ben's arrival in the FMA universe. **_


	2. Chapter 2

_**Disclaimer: I do not own Ben 10 or Fullmetal Alchemist. Ben 10 belongs to Man of Action, while FMA belongs to Hiromu Arakawa.**_

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_**Bellwood**_

A young man with brown hair, green eyes, wearing black sneakers, jeans, a black t-shirt, and a green jacket with a white stripes and a big number 10 on the right side, was sitting at a bench at his famous eatery Mr. Smoothie's enjoying a nice cool smoothie. This boy was sixteen year old Ben Tennyson, better known to many as Ben 10, wielder of the powerful alien device called the Omnitrix, and Hero of the Universe many times over. Even though he was by himself, he enjoyed it. It wasn't so often that he could go outside and not be swarmed by fans. He did not want to admit it, but he didn't like being famous, it offered him little privacy. As he finished up his smoothie, his Omnitrix was receiving a call.

"Hello?" he answered.

_"Ben. I understand this is your day off from Plumber assignments, but should you really be so relaxed?" _It was Ben's partner, Rook Blonko.

"It's cool Rook don't worry about it." Ben stated

_"I fail to understand,"_ Rook questioned, _"What if there's a universal threat or a robbery in progress?"_

"Hey you're on monitor duty, if the Plumbers find something big, you and Grandpa Max now to call me in." Ben told him.

_"Yes, true. But…"_ Rook tried but was interrupted.

"Rook, trust me. Crime here in Bellwood has hit its lowest. I know that we should always be on the look-out for the 'universal threat of the week' or something; but you gotta learn to relax, danger's not going show up every second of every day."

_"Yes. I suppose that is true,"_ Rook admitted.

"Hey partner, let me give you some helpful advice." Ben stated "Just learn to switch-off. You are a great plumber, and an awesome partner. But you have to learn to just turn off your 'Plumber's instinct' and just go with the flow."

_"Perhaps you are right,"_ Rook agreed. _"I'm sorry to have interrupted. I'll let you get back to your time,"_ Rook then signed off.

Ben was about to get up and order his fourth smoothie, but before he could; a white orb materialized right in front of him. Startled, Ben fell on his back. "Where did you come from?!"

The orb just remained floating in front of him, with a humming noise emanating from it. Not sensing any danger, Ben got up. He got closer, with his Omnitrix at the ready in case this object was dangerous. Once he was a few inches away from the orb, it suddenly exploded and Ben screamed as he was engulfed by a bright flash of light.

The light had finally subsided, though he was still seeing spots. He rubbed his eyes as he waited for them to adjust. Once the spots were gone, Ben opened his eyes. His eyes went wide in surprise once he saw that he was now in some alleyway that he had never seen before in his life, it was cloudy with the rain pouring heavily compared to the nighttime back in Bellwood, and all of this had happened in a split-second.

"Where…where am I?" Ben asked himself. He made his way out of the alley, holding up a hand to shield his eyes from the glaring sunlight as he took a look around. It appeared like he was in some city that he didn't recognize.

Ben tried to contact his friends and plumbers through his Omnitrix.

"Hey Gwen, Kevin, are you there?" there was no reply. " Gwen? Kevin? Rook? Grandpa Max are _you_ there?" he had received nothing. "Omnitrix, try connecting to the Plumber's network." The Omnitrix then beeped for a few seconds.

_"Error, network not found," _The watch replied.

"'Not found'?!" Ben repeated. "Ugh, don't tell me I got zapped to another parallel Earth."

Ben grabbed his head out of frustration and fell to the ground on his back. He looked up at the cloudy sky of where he was now stuck in, still trying to process what just happened.

Realizing that there was no use dwelling on it, Ben then raised his wrist up, the young man played with the Omnitrix for a few moments before a green holograph appeared.

Cycling through the various icons available to him, the young man soon found the one he was looking for and tapped on it. Just then, a small, black core appeared from the middle of the device. Without even hesitating, Ben slammed the core inside, causing the familiar emerald light to surround him.

'_Well, if there's one thing to be happy about, the watch hasn't changed on me._' Ben thought, realizing that this was better than nothing.

Once the light died down, instead of Ben, he was replaced by a strange creature. He was now a red, humanoid, manta-ray like alien with two, black lightning bolt-like stripes coming down from either shoulder, and converging on the familiar green hourglass symbol on his chest. It had yellow lips, which connected to yellow horns right above it's eyes that somewhat resembled large eyebrows pointed upwards.

The alien spread his arms, revealing the yellow patagia underneath them.

"JETRAY!" the creature yelled.

Crouching down, Jetray soon launched himself into the air, hoping to get a bird's eye view of the scene around him. Just then, Jetray noticed a large explosion off in the distance.

"Huh?"

Another explosion sounded out, and he could see a large dust cloud and rocks flying.

It definitely looked like something that needed checking out.

"Man, what did I get myself into?" the manta-ray-like alien said in an exasperated tone. "Oh well, looks like it's Hero Time!"

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_**East City**_

Edward Elric stared up in defeat at his would be killer.

It was just minutes after he and his younger brother Alphonse had gone out in the rain to reflect after what happened with Nina that they were approached by a dark skinned man wearing sunglasses with an x-shaped scar on his face.

He questioned whether Ed was the famed Fullmetal Alchemist, and once the blond alchemist confirmed it, a battle shortly escalated.

The fight was short, painful, and brutal. It resulted in the Elric brothers being cornered in a random alleyway with Alphonse's armor being shattered on his left side and Edward's automail arm destroyed, rendering them both defenseless from the man's wrath.

"If… If you kill me, would you let my brother go?" Edward asked, already accepting his inevitable death.

"Brother!" Alphonse yelled, horror struck, unable to move his armored body.

"The only one receiving God's wrath is you, Fullmetal Alchemist," the man replied, cryptically. "As long as your brother doesn't interfere, I'll let him go."

Ed sighed in an odd mixture of relief and defeat. Even if he dies, at least Al would be safe.

Alphonse was desperately screaming for his older brother to move, to run away. He was so scared! He couldn't bare to lose the only family member he had left!

"Back off creep!"

Suddenly, a stream of multi-colored energy shot at the dark skinned man, causing him to jump back, preventing him from delivering a killing blow on Edward.

All three of them turned towards the sound of the deep gravelly voice to see to their utter shock and disbelief, a one-eyed creature made out of a purple and magenta crystal approaching the scene.

"What in the world?!" Alphonse exclaimed, shocked.

"What the hell are you doing?! Get out of here! It's dangerous!" Edward yelled, angrily, most likely born from worry over their apparent savior's safety.

Chromastone stood in front of him, protectively. "No way I'm gonna stand by and let this guy kill you."

the Crystalsapien glanced behind him and vaguely and his eye widened for a fraction of a second at the blonde kid's missing mechanical right arm and the armored guy's half destroyed body which looked empty on the inside, '_Okay, that's weird.'_

"You should listen to the boy and run away, creature," the man hissed. "If you interfere, I would not spare you from God's wrath."

Chromastone gave the assailant a half smile. "Sorry pal, but I can't do that."

"Don't be stupid," Ed shouted. "He'll kill you!"

Chromastone took an involuntary step forward, staring the man down. "Then I'd just have to try my best to make sure he doesn't."

"So…?" The man glared at the crystal creature through his sunglasses. "You are planning to protect the Fullmetal Alchemist?"

"I said before, you won't kill anybody on my watch." Chromastone reiterated, determinedly.

Without a second to spare, the scarred man charged at the silicon alien, right arm stretched, charging blue with an unknown force. Chromastone barely reacted fast enough to dodge his attack, and watched in slight bewilderment as it exploded the ground. At least now he knew what was the cause of all those explosions.

He swiftly twisted his arm around, preparing to blow the stunned creature to pieces. On instinct, Chromastone slapped the Omnitrix symbol, transforming into an armored creature that resembled an oven. the man recoiled in pain since he felt a searing heat as soon as he touched the armor.

"NRG!"

"What is this?" the man hissed, eyeing the armored being critically as he clutched his burned hand. "Are you an alchemist?" He didn't even see it use a Transmutation Circle or clap his hands together, like the Fullmetal boy.

"No, but if I told you what it was, you'd probably wouldn't get it," NRG answered, truthfully. Immediately, the light behind his visor grille lit up before a beam of power burned through the air and seared towards the scarred man. His eyes widened before he ducked out of the way, allowing the beam to continue on and collide against the side of a building at the end of the alley. Rather than the bricks blowing up like one would expect, they actually melted from the beam which NRG had fired. Seeing he missed, the armored alien cut the flow of power in favor of eyeing the scarred man again.

NRG eyed him dangerously. "Now you're going to stand down and surrender buddy. If you cooperate, I'll make sure you only get off with a light tan." He accentuated the threat by heating up his gauntlets, making them turn red from the temperature increase.

The man glowered at the creature. Was he mocking him?

A gunshot cracked the still air which caused the scarred man, who had stopped in his tracks, to look off into the distance.

"That's enough!" A deep voice echoed off the empty streets. NRG looked up for its owner, to find an entire row of state soldiers in blue, guns drawn, pointing their firearms in their direction. "You wont be killing anyone else today, Scar." the black haired man spoke confidently, his gun still in the air. "I'm taking you into custody, where you will answer for the murders of at least ten State Alchemists."

"Alchemists alter things from their natural form, perverting them into something else, something grotesque." Scar said matter of factly, his eyes darting to NRG. "They profane God, the true creator of all things. As an agent of God I'm here to hand down his judgment." he spoke, not taking his eyes off of the crature. "If you interfere I will eliminate you as well."

"Oh, is that right?" The soldier said haughtily. He handed his gun off to the woman beside him, who took it with wide eyes.

"Colonel Mustang!" The woman called after him. The Colonel ignored her and NRG watched as he strode out into the middle of the street confidently. Scar narrowed his eyes, amused.

"Colonel Mustang? So this is the Flame Alchemist." A look of realization came over Scar's face and he stepped away from NRG as he watched him walk over to the Colonel, ready to fight. "To think you would volunteer yourself for my judgment. This is truly an auspicious day!" Scar began to run at him, but the colonel kept his same pace.

"So you know who I am and you still want to fight?" The Colonel said holding his hand up in front of his face. The woman from before ran up behind him and just as the Colonel went to snap his fingers, she tripped him. Scar's hand missed the Colonel's face by mere inches and the woman drew two side arms, firing them in earnest at the psychopath who'd nearly killed her superior.

NRG watched in slight amazement as he danced around the bullets, each of them hitting in the dirt at his feet, or embedding themselves in building behind him. Scar drew closer to them as he dodged the bullets, and NRG's eyes widened. '_Oh no, he's coming back towards us!'_ he glanced over at the one called Fullmetal, as he too was thinking the same thing.

Channeling his heat, NRG slammed both of his hands into the ground. Everyone's mouth hanged wide open as massive geysers of lava erupted from the ground towards Scar. Roy felt another blow to his pride, some stranger he's never met was able to produce heat in the rain, and from the ground too! But… oddly enough, Roy didn't see any kind of transmutation circle on his armor or palms which peaked his interest of whoever this newcomer was. Of course, Scar barely got away by creating a massive hole in the street ignoring the burning sensation in his hand as he did, but not before everyone could see the red of his eyes, revealing his Ishvalan heritage for the world to see.

NRG looked over to Ed while the military acted like morons, watching as the young alchemist ran to his broken brother in worry, trying to get him to talk.

"Al? Talk to me, are you alright? Al? Al?" Everyone tensed when Alphonse suddenly punched his brother in the face, sending Ed backwards in an almost comical fashion. NRG could have sworn that Ed's body looked like a rag doll after getting punched liked that.

"Why didn't you run away when I told you to?! What kind of idiot are you?!"

"No way! I'm not going to run away and leave you behind!"

"Which is exactly why you're an idiot!" Rage emanated from the armor's very fists as he drove another punch to his brother. How is the kid still alive?

"Why do you keep punching me like that? If I had run away you could have been killed, you know that?"

"Maybe I wouldn't have been! But making the decision to die is something only an idiot does!"

"Hey easy with the idiot stuff! I'm still your older brother got it!"

"I'll say it all I want to!" Al grabs Ed by the shirt, yanking his older brother's face closer to his, "Survival is the only way Ed. You're the only one who can restore us to our original bodies. You have to live on, learn more about alchemy. You can help so many lives, lives like Nina if you did! I won't allow you to abandon the possibility of hope and chose a meaningless death!" As if the world had perfect timing, Al's arm broke off and fell to the ground with a series of clangs.

"OH GREAT! And now my arm's come off because my brother's a big, fat, idiot!"

Ben inwardly sighed as he relaxed his posture, but the military seemed to have different ideas since they now had their guns aimed at the Omnitrix wielder.

Mustang frowned at the armored giant, partly in displeasure by the fact that Scar had gotten away again, partly in wariness of the stranger in front of him, "I like to thank you for saving Fullmetal and his brother, I am Colonel Roy Mustang of the Amestrian military," The colonel said.

A bespectacled man suddenly appeared from out of nowhere, "Oh, is it over?"

"Lieutenant Colonel Hughes, where have you've been this whole time?" Demanded Major Armstrong.

"I thought it best to lay low." Hughes replied.

"You didn't think of maybe backing us up?!" Shouted Mustang.

"Of course not! A person like me shouldn't get dragged into a freak show with a pack of you pseudo-humans! It's bad for my health!" Hughes turned to face the soldiers standing by. "Don't just stand there! Deploy troops! Circulate his description!"

"Yes sir!"

Mustang then turned to NRG, "Anyway, we should talk more at headquarters. I'm sure we all have some questions for you, may I ask what your name is?"

Ben figured there was no point in keeping his identity secret as he was in unknown territory and he didn't want to give these soldiers any reason to consider him a hostile threat. He mentally groaned, might as well get this over with.

"Well, first things first," NRG said as he raised his hand and touched the Omnitrix symbol on his chest. A green light enveloped the armored man, blinding everyone present. When the light faded, NRG was replaced a teenage boy with brown hair and green eyes wearing a green jacket and jeans.

"You can call me Ben Tennyson," the young hero finished as the Elric Brothers, Mustang, Hawkeye, Hughes, Armstrong, and the other soldiers all showed various levels of surprise and shock.

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_**AN: Alright, let's stop here for now, I hope you enjoyed this chapter. If you have any suggestions for how I should proceed with this story, I'm all ears. Until then, I'll see you next chapter. **_


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